Immunological mechanisms in the pathogenesis of rheumatoid arthritis may be revealed by study of the remarkably dense lymphocytic infiltrations in rheumatoid synovial membranes. A method was developed for extraction of viable lymphocytes by digestion of synovium with collagenase and deoxyribonuclease and B plus T cells quantitated in these populations. Adherence of C sensitized erythrocytes to C3 bearing B cells in cryostat tissue sections were also examined. Functional aspects of the lymphocytes will also be examined.